PROBLEM TO BE SOLVED: To provide a boundary detector capable of improving boundary detection accuracy.SOLUTION: Binarization processing using a region expansion method and binarization processing using threshold processing are performed for image data of each slice to obtain two kinds of binarization image data. Next, an area A of a region to which a logical value 1 of the binarization image data is assigned, and an area B of a region to which the logical value 1 of the binarization image data is assigned are obtained, and an area ratio R of the area A to the area B is obtained. Then, from a graph showing the relationship between each slice and the area ratio R, a position where the area ratio R changes steeply is detected to obtain a position of the boundary of a lung and a liver.
